Nitride semiconductors such as gallium nitride (GaN) and related semiconductors are widely regarded as desirable wide bandgap compound semiconductors. These materials have been adopted in optoelectronic devices such as light-emitting diodes (“LEDs”), laser diodes and photodiodes, and have also been employed in non-optical electronic devices such as field effect transistors (“FETs”) and field emitters. In optoelectronic devices, the wide bandgap of the material allows for emission or absorption of light in the visible-to-ultraviolet range. In electronic devices, GaN and related materials provide high electron mobility and allow for operation at very high signal frequencies.
In some applications, GaN materials are grown on a substrate. A silicon (Si) substrate, for example, is relatively inexpensive for growth of a GaN layer. A Si substrate not only has the advantages of low cost and good electrical and thermal conductivity, but also is available in larger wafer size. Further, GaN epitaxy on Si facilitates integration of microelectronics and optoeleetronics. However, it is difficult to grow single crystal GaN directly on a Si substrate because of large lattice and thermal mismatches between GaN and Si.
Likewise, differences in the lattice constant between GaN materials and other substrate materials can lead to difficulties in growing layers suitable for many applications. The difference in lattice constant may lead to the formation of defects in GaN material layers deposited on substrates. Such defects can impair the performance of devices formed using the GaN material layers.
Use of thin interlayers with in-plane lattice constants smaller than the bulk GaN material has been used to engineer the lattice and thermal mismatch of the bulk GaN layer and the Si substrate in order to obtain epitaxial growth of crack free GaN on a Si substrate. However, because the epitaxial grown interlayer with smaller in-plane lattice constants exhibits a compressive strain to the bulk GaN layer, an undesired two-dimensional electron gas (2 DEG) can be created at the interface of such interlayer and the GaN material.